Titanium dioxide pigment composition and a method for making the same

ABSTRACT

An aluminum silicate encapsulated pigmentary titanium dioxide combining both high tinting strength and durability in coating compositions and in particular thermosetting and thermoplastic acrylic coating compositions is prepared by coprecipitating hydrous silicon oxide with hydrous aluminum oxide onto titanium dioxide in aqueous slurry in a manner to form a dense skin coating of aluminum silicate, optionally applying a sponge coating of aluminum oxide and thereafter finishing the silicate coated pigment by washing, drying and milling.

United States Patent Foss 5] Mar. 14 1972 [54] TITANIUM DIOXIDE PIGMENT 3,437,502 4/1969 Werner ..l06/300 COMPOSITION AND A METHOD FOR 3,497,373 2/1970 Rieck etal .106/300 X MAKING THE SAME 3,510,334 5/1970 Goodspeed". ....l06/300 3,515,566 6/1970 Moody et al ..106/300 [72] lnventor: Warren M. Foss, 227 Woodbridge Ave,

Metuchen, NJ. 08840 Primary ExaminerTobias E. Levow Assistant Examinerl-l. M. S. Sneed 22 F1 A 0 0 1 led pr 2 197 AttorneyCharles F. Kaegebehn, Robert L. Lehman, Harold [21] Appl. No.: 30,219 L. Gammons and Robert L. Holiday Related US. Application Data 57] ABSTRACT [63] (ljggtinuationin'pan of July An aluminum silicate encapsulated pigmentary titanium dioxide combining both high tinting strength and durability in coating compositions and in particular thermosetting and {$8.81 "10600068364382 thermoplastic acrylic coating compositions is prepared by l 58] Field 00 2 B coprecipitating hydrous silicon oxide with hydrous aluminum oxide onto titanium dioxide in aqueous slurry in a manner to form a dense skin coating of aluminum silicate, optionally ap- [56] References Cited plying a sponge coating of aluminum oxide and thereafter UNITED STATES PATENTS finishing the silicate coated pigment by washing, drying-and millin 3,035,966 5/1962 Siuta ..106/300 X g 3,383,231 5/1968 Allan ..l06/300 13 Claims, NoDrawings TITANIUM DIOXIDE PIGMENT COMPOSITION AND A METHOD FOR MAKING THE SAME The silicate encapsulated TiO may be finished in any one of several ways depending upon the number of silicate coatings applied. Thus when a single skin coating of aluminum silicate is applied to the pigment the latter will be finished by applying a This application is a continuation-in-part of my application Ser. No. 839,088 filed July 3, 1969 now abandoned.

BACKGROUND OF THE INVENTION Among the well-known uses of 'IiO pigments is its use in coating compositions as, for example, in linseed oil base paints and in resinous systems such as paints having an alkyd resin base. These prior art pigments have been designed to impart high opacity, good gloss, high chalk resistance and other desirable properties to the aforesaid paint systems for the particular purpose desired. However, industry is now showing considerable interest in the more durable acrylic resin systems but it was found, at the outset, that TiO pigments which are acceptable in linseed oil or alkyd systems were unsatisfactory in acrylic systems.

SUMMARY OF THE INVENTION in its broadest aspects the invention relates to pigmentary TiO encapsulated with an impervious dense skin of a hydrated aluminum silicate with or without a coating of sponge alumina the amount of hydrated aluminum silicate on the TiO pigment being from 5 to 20 percent on a TiO weight basis and the amount of sponge alumina being from 1.0 to 2.0 percent.

While an exact explanation for the superior results achieved in acrylic systems by the silicate encapsulated Ti0 pigment of this invention is not known, it is postulated that the nature of acrylic resins, as distinguished from linseed oil or alkyd systems, is such that it is imperative that the pigment be completely shielded from contact with the acrylic resin; and that the coating used to shield the TiO be one which is substantially impervious. Heretofore, it has been assumed that when a TiO pigment is treated with hydrous oxides i.e., titania, alumina, silica or the like, using the techniques of the prior art, the hydrous oxide(s) provided a uniform coating over the entire surface of the pigment. It has now been shown by electron photomicrographs that this is not generally so but that the coatings are discontinuous and/or porous and, as a consequence, relatively large areas of the pigment are exposed.

Pursuant to the objects of the invention, it was discovered that when the hydrous oxides of aluminum and silicon are coprecipitated onto pigmentary TiO in the manner hereafter described, a dense skin of hydrated aluminum silicate is formed which completely encapsulates the TiO pigment particles and, as a consequence, the treated pigment is characterized by excellent durability in acrylic resin coating compositions. The dense skin coating of aluminum silicate may be applied in a single stage in which case a final source coating of aluminum oxide is necessary to insure a pigment having both good durability plus good tinting strength; or optionally the skin coating of aluminum silicate may be applied in two or more successive stages preferably with drying and milling following each successive stage, in which case the final pigment will combine excellent tinting strength with excellent durability. The invention also embraces the addition of a final sponge coating of aluminum oxide to multiple skin coatings of aluminum silicate to form a pigment which combines maximum tinting strength with maximum durability in acrylic systems.

As will be apparent from the description which follows, the term encapsulated as used herein and in the claims is definitive of a continuous unbroken coating completely enclosing the TiO pigment, while the term dense" as applied to the coating is indicative of its imperviousness. Also, inasmuch as the amount of silicate used to encapsulate the TiO pigment is relatively small, the coating is relatively thin and by the nature of its application adheres tightly to the pigment and hence may be further identified as a skin coating. In contradistinction the term sponge," as used hereinafter to describe the coating of hydrous aluminum oxide, is indicative of the relatively fluffy, pervious hydrous oxide coatings heretofore commonly used in the art.

The process of this invention may bedescribed briefly as comprising encapsulating pigmentary TiO, in a dense skin of hydrated aluminum silicate in one or two stages in which SiO, and A1 0 are coprecipitated onto the pigmentary TiO, by forming an aqueous slurry of pigmentary TiO heating the slurry to a temperature of at least 70 C., adjusting the pH to within the range of 8.5 to 10.5, then simultaneously and slowly adding solutions of soluble salts of silicon and aluminum to form an aluminum silicate coating while maintaining the pH of the slurry from 8.5 to 10.5 and at a temperature of at least 70 C. Whether applying a single skin coating ofaluminum silicate or multiple skin coatings each coating of hydrous aluminum silicate precipitated on the TiO in aqueous slurry is allowed to age for a predetermined length of time.

The silicate encapsulated TiO, may be finished in any one of several ways depending upon the number of silicate coatings applied. Thus when a single skin coating of aluminum silicate is applied to the pigment the latter will be finished by applying a sponge coating of alumina followed by drying at a temperature from 120 C. to 250 C. and milling, preferably at a steam to pigment ratio of from 3:1 to 5:]. However when multiple skin coatings of aluminum silicate are applied to the pigment it is preferred, though not essential, to finish the pigment, following one or more skin coatings by drying the pigment at from 120 C. to 350 C. and then milling by steam micronizing at a steam to pigment ratio of from 3:1 to 5:1. Moreover for maximum tinting strength and durability the multicoated pigment may be given a final sponge coating of alumina followed by drying at 120 C. and milling.

Although the silicate encapsulated'TiO pigment of the instant invention exhibits acceptable properties when used in oil or alkyd resin systems, it has been found that when used in acrylic systems it combines both high hiding power with high durability and/or resistance to chalking.

DESCRIPTION OF THE PREFERRED EMBODIMENT The starting material for the preparation of the pigment of the instant invention is a finely divided TiO pigment which may be raw or unprocessed TiO produced by the vapor phase reaction of TiCl, with oxygen; or a raw vapor phase TiO which has been calcined to remove the chloride values. Also, the pigment'may be a calcined TiO, prepared from hydrolysis of a titanium sulfate-ferrous sulfate solution. In any case, however, it is necessary to employ a TiO possessing good color, i.e., high lightness and tone as measured by the green, red and X-blue reflectance values on a Colormaster Differential Colorimeter.

The Ti0 pigment is slurried in water at 15-30 percent solids, heated to a temperature from at least 70 C. to boiling and the pH of the slurry adjusted to between 8.5 and 10.5 by adding an alkalizing agent, such as NH,OH or NaOH. While the slurry is held within this alkaline pH range, salt solutions of silicon and aluminum are added simultaneously and slowly, i.e., at a rate not to exceed 10 percent total silicate coating per hour, to coprecipitate A1 0;, and SiO onto the surface of the pigment particles as a dense, silicate skin of hydrated aluminum silicate. The dense silicate skin is then aged by maintaining the pH of the slurry at 7.0-l0.5 while holding the slurry at a temperature of at least 70 C. to boiling for at least 45 minutes to minutes. When two or more skin coatings of aluminum silicate are to be applied then the pH of the aged slurry is readjusted, if necessary, to 8.5-10.5 prior to applying the next coating by adding an alkalizing agent whereupon another dense skin coating of hydrated aluminum silicate is precipitated in the same manner as the preceeding silicate coating, each succeeding skin coating of aluminum silicate being aged by holding the pH of the slurry at 7.0-10.5 and at a temperature of at least 70 C. to boiling for at least 45 minutes to 90 minutes. Following application of the final skin coating of aluminum silicate the slurry is filtered and the silicate encapsulated TiO washed, dried by heating to an elevated temperature, in accordance with the conditions stipulated above, followed by micronizing at a steam to pigment ratio of from 3:l to 5:1.

As stated above, the SiO and A1 are coprecipitated by simultaneously adding solutions of soluble salts of aluminum and silicon to 'the slurry and it is essential to the formation of the dense skin coating of hydrated aluminum silicate that the pH of the slurry be maintained within the limits of 8.5 to 10.5, if one of the soluble salts is acidic, then an alkalizing agent is also added to maintain the alkaline pH. Moreover, despite the precipitation of the dense skin of hydrated aluminum silicate in one or two stages with ageing between stages, the final skin coating has been found to be a homogeneous, impervious, hydrated aluminum silicate which completely encapsulates the pigmentary TiO To be most effective in acrylic systems, the amount of dense hydrated aluminum silicate coating on the TiO pigment particles should be from percent to 20 percent based on the weight of the TiO Moreover, the ratio of Si0 to A1 0 may range from 2:1 to as high as 8:] but is preferably about 4:1. Further, it is not necessary that equal amounts of silica and alumina be coprecipitated at each stage and equally effective results have been achieved by coprecipitating from to $41 of the total amount of Si0 and A1 0 in the first stage, and the remaining portion in the second stage.

As mentioned at the outset, while a pigmentary TiO- encapsulated with a dense impervious skin of hydrated aluminum silicate according to the multistage coating process outlined above has outstanding optical properties plus durability when employed in acrylic resin systems, it has been found that when the silicate skin coating is formed by the use of relatively low amounts of silica and alumina, either as a single skin coating or multiple skin coatings, the durability of the silicate coated pigment is further enhanced by coating the silicate encapsulated TiO with sponge alumina in the following manner. Thus,

the silicate encapsulated TiO while still in the treatment slurry is further treated by lowering the pH to 5.0-5.5 with dilute sulfuric acid while holding the temperature between 10 C.100 v c. and preferably at 90 c. A water soluble salt Giantminum, i.e., sodium aluminate and/or aluminum sulfate,as the case may be, in the treatment range of 1-2 percent, based on the weight of the TiO is added slowly over a 30 minute addition period while the pH is held constant at 5.0- and the temperature of the slurry at 90 C. After minutes the pH is adjusted to a pH of 78 and maintained at this level until all of the soluble aluminum salt is precipitated as spongelike hydrous aluminum oxide on the silicate encapsulated TiO Thereafter, the slurry is filtered to recover the pigment product which is washed and then heated to a temperature sufficiently high to partially dehydrate the sponge alumina, a preferred temperature being from about 120 C. to about 350 C. Although a temperature as high as 500 C. may be used. This coating of alumina is relatively fluffy and pervious in composition and is herein referred to as sponge alumina.

When the pigment is given a single skin coating of aluminum silicate the sponge alumina is applied without prior drying and milling of the silicate encapsulated pigment. On the other hand when multiple skin coatings of aluminum silicate are applied to the pigment the sponge alumina is applied preferably following drying and milling of the final skin coating ofaluminum silicate.

Moreover when the sponge alumina treatment is applicable to a silicate encapsulated Ti0 wherein the ratio of silica to alumina is relatively high, i.e., 4:1 or higher it has been found that the coating of sponge alumina is especially advantageous when the ratio of silica to alumina is relatively low i.e., about 2:1. At these low ratios, and hence relatively low amounts of silica on a Ti0 weight basis, the durability of the silicate encapsulated Ti0 tends to be relatively low. It has been found however that coating the silicate encapsulated TiO with spongelike hydrous aluminum oxide and heating at elevated temperatures sufficient to partially dehydrate the alumina, i.e., temperatures from l20 C.-350 C. the finished pigment will have high tinting strength plus durabilities equal to or better than a silicate encapsulated pigment prepared with higher ratios of silica and alumina this despite the relatively low amount of silica on a Ti0 weight basis.

The following procedures were used for testing the optical properties and durability of the silicate encapsulated TiO, pigment of this invention and for comparing it with TiO, pigments made according to the methods of the prior art.

A thermosetting acrylic paint was prepared by thoroughly mixing a pigment sample with a thermosetting acrylic resin (Rohm and Haas-Uformite MM -47), at 15 PVC with a ratio of acrylic to melamine of 70 to 30, The paint was then sprayed onto an aluminum panel (two coats), air dried for 20 minutes and then baked for 17 minutes at [20 C, The green, red and blue reflectance values of the pain panel were measured on a Colormaster Differential Colorimeter, manufactured by Manufacturers Engineering and Equipment Corp., Hatboro, Pa. The green reflectance values of the pigment samples (expressed in percent) were compared with standard pigments for brightness. The blue minus red reflectance values divided by green reflectance were taken as a measure of the color tone of the pigment samples. The tinting strength of the coating composition was measured by the standard Reynolds Tinting Strength Test as described in Physical and Chemical Exam ination of Paints, varnishes, Lacquers and Colors by H. V.

Gardner; 9th. Edition 5/39 page 37.

The paint panels were tested in an Atlas WeatherOmeter to determine their durability on exposure to atmospheric weather conditions; and were also tested for acid solubility as a measure of density of the aluminum silicate skin coating by digesting 0.5 grams of pigment in 25 ml. of 96% l'l SO for 1 hour at 175 C. utilizing an oil bath to control the temperature. it is very important that the temperature be accurately controlled to obtain reproducible results. Acid solubilities of no more than 10 percent and preferably as low as 4 percent or lower are considered to be indicative of a dense skin coating highly resistant to chalking.

The following examples are presented to show further the details of the instant invention EXAMPLE 1 The Ti0 pigment used as the starting material in this and the following examples was produced as the reaction product of the vapor phase reaction between titanium tetrachloride and an oxygen-containing gas. The TiO pigment was slurried and dispersed in water at 20 percent solids and heated to C. The pH of the pigment slurry was adjusted to 9.0 by the addition of NaOH. An N-Brand sodium silicate solution containing 60 g.p.l. SiO was then added to the slurry simultaneously with a solution of aluminum sulfate at a rate of 5 percent total treatment per hour. in order to keep the pH constant at 90, NaOH was added to the sodium silicate solution. The amount of Si0 added was 4% on a TiO basis and the amount of Al O added was 1% on a TiO basis. After the solutions were added, the pH of the treated slurry was lowered to 7.0 by the addition of H 50 A dense skin coating of hydrated aluminum silicate formed on the surface of the Ti0 and was allowed to age at 90 C. for 1 hour. After ageing, the silicate coated Ti0 was subjected to a second coating 5:1 treating the slurry with NaOH to again raise the pH to 9.0 and adding water soluble silicate and aluminum compounds in the same manner as was done in the first stage. The slurry was then treated with H 50, to lower the pH to 7.0 and the second coating of aluminum silicate allowed to age at 90 C. for 1 hour. After the second ageing process, the silicate encapsulated Ti0 was then filtered, washed and then finished by drying at l20 C. followed by milling in a steam micronizer at a steam to pigment ratio of This hydrated aluminum silicate encapsulated TiO pigment possessed a tinting strength of 1775 and an oil absorption of 26.8. When this sample was made up into a paint and subjected to a WeatherOmeter to measure the durability, its durability after 70 hours was 9 on a scale ranging from 1 to 10 with 1 rated as poor and 10 rated as excellent. This value was comparable to outstanding durability as measured by outdoor exposure tests. Further, acid solubility was 5.0 percent, the brightness of the pigment when incorporated in an acrylic vehicle was 95 (percent green reflectance) and the colortone was 4.

EXAMPLE 2 In this experiment the procedure ofExample 1 was repeated except that the pH of the slurry was held at 9.0 during both the precipitation and ageing steps. The results are recorded in the Table along with those of Example 1. Again the finished pigment combined both high tinting strength (1725) with low acid solubility (7.1 percent) i.e., high durability.

EXAMPLES 3-6 The procedure of Example 1 was repeated except that varying amounts of SiO and A1 were employed in varying ratios, the total amounts of coding agents employed in Examples 3-5 being within the ranges contemplated by the instant invention. In Example 6 however the total amount of alumina and silica coprecipitated in two stages was only 5 percent and neither high temperature drying nor sponge alumina was used to finish the pigment. lt should be noted that while the tinting strength of the silicate encapsulated pigment of Example 6 was comparable to those of the proceeding examples the acid solubility increased to 39.0 percent which is far beyond the upper permissible limit of 10.0 percent.

EXAMPLE 7 This run is presented to show that a good quality pigment is also obtained when a single skin coating consisting of a relatively low total amount of silica and alumina is applied to the TiO pigment followed by a coating of 1.0 percent sponge alumina. Thus a single skin coating of aluminum silicate was applied following the procedure of Example 1, the total amount of coprecipitated silica and alumina employed to form the single skin coating of aluminum silicate being 5 percent. The sponge alumina coating was then applied to the single skin coating of aluminum silicate in the manner described above the amount of alumina as Al. ,O being 1.0 percent, after which the pigment was dried and milled. It will be seen fro the Table that the acid solubility was 7.0 percent which is well below the upper permissible limit of 10.0 percent and that the finished pigment had exceptionally high tinting strength.

EXAMPLE 8 The procedure of Example 6 was again repeated i.e., a two stage treatment using a total of only 5 percent coprecipitated alumina and silica. However in this example the silicate encapsulated TiO pigment was dried, milled and then coated with 1.5 percent sponge alumina, using the procedure described above, followed by drying at about 120 C. to form sponge alumina and thereafter steam micronized at a steam to pigment ratio of 5:1. As shown in the Table below the pigment retained all of the desirable optical properties of the pigment of Example 6 and in addition had an acid solubility value within the acceptable range for good durability.

EXAMPLE 9 Another pigment was made using the procedure of Example 1 except that the ratio of silica to alumina was 2:1 with 6 percent total silica and alumina. Following the second stage coating the slurry was filtered and the silicate encapsulated TiO washed and then coated with 1 percent sponge alumina followed by calcination at 350 C. and steam milling at a steam to pigment ratio of 5:1. As shown in the Table the tinting strength of this pigment was excellent and its durability was comparable to that of the silicate encapsulated pigment prepared with higher amounts of silica.

EXAMPLE 10 Additional pigments were made following the two stage treatment technique of Example 1 wherein the ratio of silica to alumina was 4:1 in each stage. However following ageing of the first skin coating the pigment slurry was filtered and the silicate encapsulated TiO, pigment was washed, dried at 120 C. and milled in a steam micronizer at a steam to pigment ratio of 5:1. Thereafter the TiO reslurried, the pH of the slurry again adjusted to 9.0 and the second skin coating of aluminum silicate was applied which in turn was aged and thereafter the slurry was filtered followed by washing, drying the encapsulated TiO pigment at 120 C. and then steam micronizing at 5:1.

As shown in the Table the finished pigment had a tinting strength in acrylics of 1735; a brightness of 95.0 and an oil absorption of 19.7 and a durability measured in terms of H solubility of 2.5. This pigment thus combined excellent tinting strength with excellent durability.

EXAMPLES 11 AND 12 Two additional pigments were made again using multiple skin treatments following the procedure of Example 1 except that the ratio of silica and alumina was only 2:0.25. Thus the total amount of silica and alumina used was only 4 percent and 0.50 percent respectively. As in Example 10 the silicate encapsulated pigment was dried and steam micronized following each skin treatment. Thereafter the pigment was given a sponge coating of 1.5 percent alumina after which it was dried at C. and steam micronized at a steam to pigment ratio of 5:1. The pigment of Example 11 differed from the pigment of Example 12 in that each skin treatment of Example 11 was dried at 120 C. whereas in Example 12 the first skin coating was dried at 300 C. and the second at 120 C.

As shown in the Table each of these silicate encapsulated pigments combined exceptionally high tinting strength with maximum durabilities.

EXAMPLE 13 An additional experiment was run to show the superiority of the aluminum silicate encapsulated TiO ofthis invention over pigments prepared according to the prior art. The pigment in this example was prepared according to the teaching of US. Pat. No. 3,383,231 wherein TiO is coated with a mixture of alumina and silica at relatively low pH as a consequence of which the alumina-silica coating was fluffy and pervious. As shown in the Table below this prior art pigment had good tinting strength but very high (32.8 percent) solubility in H 80 and hence did not combine the high durability and excellent optical properties that characterize the improved pigment of this invention.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other variations and modifications may be em ployed within the scope of the following claims.

TABLE Treatment Example no.

First silicate coating Slurry pH 7: SiO

71 A1 0 Drying (CJ Milling (s/p) Second silicate coating Slurry pH 9 9 it SiO 4 4 l l l (\VeulheFOmcter) 9 9 a so. solubilitytS 5.0 7.1 4.2 3.2 Brightness (/t green reflectance) 95.2 94.6 94.4 94.4 Color Tone /r XB-i RFIL; +4.0 +4.7 +4.7 +4.9 Oil Absorption 26.8 28.3 28.7 21.8

5 6 7 3 First Coating Slurry pH 9 9 9 SiO, 3 2 4 2 Q A1 1 (1.5 l 0.5 Drying (C-l Millingis/p) Second Coating Slurry pH 9 9 9 $1 510 3 2 0 2 A1 0, 1 0.5 0 0.5 Drying (c1 I20 120 120 Millingts/pl 5.1 5:1 5:1 Sponge Al. .O Coatinglk] t) 0 1.0 1.5 Drying|jC.l 120 120 M illingls/p 5:1 5:1 Properties Timing Strength 1725 1725 1850 1825 H 50, Solubility (A1 9.0 39.0 7.0 10.0 Brightness (.1 green reflectance) 94 3 95 6 93.9 95.1 Color Tone ('ZXB-ARH'rlG +5.0 +5.0 +5.1 +5.1 Oil Absorption 24.0 11.3 18.5 23.5

10 ll 12 13 First Coating Slurry pH 9 9 9 9 3-8 SiO, 2 4 2 2 4 .i A1 0 1 1 0.25 0.25 1 Drying 1C.) 120 120 300 Millingts/p) 5.1 5:1 5:1 Second Coating Slurry pH 9 9 9 9 34; /1 SiO-; 2 4 2 2 4 1i A1 0 l l 0.25 0.25 1 Dryingt C.) 120 I 120 Millingts/p) 5:1 5:1 5:1 Sponge A1 0: coatingfii) 1.0 1.5 1.5 0 Drying ("C1 350 120 I20 MillingH/p) 5.1 5:1 5:] Pro erties Tinting Strength 1875 1735 1875 I900 I825 H- -S0 Solubilitylfi) 3.1 2.5 4.7 1.5 32.8 Brightness (/1 green reflectance) 95.1 95.0 94.2 94.7 94.9 Color Tone XB-Q R/XG +5.1 4.8 5.7 5.9 +3.9 Oil Absorption 24.0 19.9 20.3 20.3 38.3

I claim:

1. A silicate encapsulated TiO consisting of pigmentary TiO having an impervious, dense skin of hydrated aluminum silicate in an amount from 5.0 percent to 20.0 percent on a TiO weight basis, and optionally a coating on said aluminum silicate skin of from 1.0 to 2.0 percent sponge alumina calculated on a TiO weight basis.

2. A silicate encapsulated TiO pigment according to claim 1 wherein the amount of sponge alumina on said silicate encapsulated T10 is about 1.5 percent calculated on a TiO weight basis.

3. A silicate encapsulated TiO pigment according to claim 1 wherein the silicate encapsulated TiO pigment has been dried and milled.

1. A silicate encapsulated Ti0 pigment according to claim 3 wherein the amount of hydrated aluminum silicate on said 'l'iO pigment is from 5-10 percent and the amount of sponge alumina on said silicate encapsulated TiO is about 1.0 percent calculated on a TiO weight basis.

5. A method for producing a silicate encapsulated TiO pigment characterized by a dense impervious skin of hydrated aluminum silicate and having exceptionally good color and durability when incorporated in acrylic coating compositions which comprises forming an aqueous slurry of pigmentary T10 adjusting the pH of the slurry to fall within the range of from 8.5 to 10.5 and coprecipitating SO; and A1 0 onto the pigmentary TiO in two successive stages to form successive coatings of hydrated aluminum silicate on said pigmentary TiO the first stage coprecipitation of SiO, and A1 0 being effected by heating the slurry to a temperature of at least 70 C. to boiling, simultaneously adding solutions of soluble slats of silicon and aluminum to said slurry at a rate not to exceed 10 percent total silicate coating per hour on a TiO weight basis and maintaining the pH of said slurry in the range 8.5 to 10.5 and the temperature of said slurry at least 70' C. to boiling during the addition of said solutions, thereafter adjusting the pH of the slurry in the range of from 7.0 to 10.5 and maintaining the slurry at a temperature of at least 70 C. to boiling for at least 45 to minutes to age the hydrated aluminum silicate coating on said pigmentary TiO the second stage coprecipitation of SiO and A1 0 being effected by readjusting the pH of the said slurry in the range of from 8.5 to 10.5 and again simultaneously adding solutions of soluble salts of silicon and aluminum to said slurry in the same manner as that described for the first stage coprecipitation. thereafter read justing the pH of the slurry at a temperature of at least 70 C. to boiling for at least 45 to 90 minutes to age the second coating of hydrated aluminum silicate, the amount of soluble salts of silicon and aluminum added to said slurry being such that the total amount of hydrated aluminum silicate deposited on said pigmentary TiO is from 5 percent to 20 percent based on weight of said TiO whereby said pigmentary TiO is completely encapsulated by said hydrated aluminum silicate, said soluble salts being added in proportions to provide a ratio of SiO :Al O in the range from 2:1 to 8:1 and thereafter finishing the silicate encapsulated T10 by drying at from C. to 350 C. followed by milling.

6. A method for producing a silicate encapsulated TiO pigment according to claim 5 wherein the temperature of the TiO;, slurry is maintained at 90 C. during both said first and second stage coprecipitations of the alumina and silica and each coating of hydrated aluminum silicate is aged for 45 minutes.

7. A method for producing a silicate encapsulated TiO pigment according to claim 5 wherein the total amount of silica and alumina used in forming said skin of hydrated aluminum silicate is 10 percent on a 'liO weight basis and the silicate coated TiO is dried by heating to a temperature of 120 C, and steam micronized at a steam to pigment ratio 0f5: l.

8. A method for producing a silicate encapsulated TiO pigment according to claim 5 wherein the total amount of silica and alumina used in forming said skin of hydrated aluminum silicate is 5 percent on a TiO weight basis and the silicate encapsulated TiO is finished by drying at a temperature from 120: 5 0f C.. steam micronizing at a steam to pigment ratio of from 3:1 to 5:1, and thereafter coated witlffrom 1.0 to 2.0 percent hydrous aluminum oxide on a TiO weight basis, followed by drying and steam micronizing.

9. A method for producing a silicate encapsulated 'l'iO pigment according to claim 5 wherein the ratio of silica and alumina used in coprecipitating the hydrated aluminum silicate on said pigmentary TiO is 8:1 the total amount of silica and alumina comprises about 4.5 percent on a TiO weight basis, and the silicate encapsulated pigment is finished by drying at a temperature of 120 C., steam micronizing at a steam to pigment ratio of 5:1 and thereafter coated with 1.5 percent hydrous aluminum oxide on a 'l'iO weight basis followed by drying at 120 C. and steam micronizing at a steam to pigment ratio of 5: 1.

10. A method for producing a silicate encapsulated TiO pigment according to claim 5 wherein the ratio of $10 to A1 0 is 4:1 and the application of each successive skin coating is followed by drying and milling.

11. A method for producing a silicate encapsulated TiO, pigment according to claim 10 wherein each successive skin coating is finished by drying at a temperature within the range from 120350 C. and milling at a steam to pigment ratio of from 3:1 to 5:1.

12. A method for producing a silicate encapsulated TiO pigment according to claim 11 wherein the ratio of SiO to A1 is 8:1 and a coating from 1.0-2.0 percent hydrous aluminum oxide is applied to the silicate encapsulated pigment following drying and milling of the final skin coating after which the alumina coated silicate encapsulated pigment is dried and milled.

13. A method for producing a silicate encapsulated TiO pigment characterized by a dense impervious skin of hydrated aluminum silicate and having exceptionally good color and durability when incorporated in acrylic coating compositions which comprises forming an aqueous slurry of pigmentary TiO adjusting the pH of the slurry to fall within the range of from 8.5 to 10.5 and coprecipitating SiO and A1 0 onto the pigmentary TiO to form a coating of hydrated aluminum silicate on said pigmentary TiO the coprecipitated of SiO and A1 0 being effected by heating the slurry to a temperature of at least 70 C. to boiling, simultaneously adding solutions of soluble salts ofsilicon and aluminum to said slurry at a rate not to exceed 10 percent total silicate coating per hour on a TiO weight basis and maintaining the pH of said slurry in the range 8.5 to 10.5 and the temperature ofsaid slurry at least 70 C. to boiling during the addition of said solutions, thereafter adjusting the pH of the slurry in the range from 7.0 to 10.5 and maintaining the slurry at a temperature of at least 70 C. to boiling for at least 45 to minutes to age the hydrated aluminum silicate coating on said pigmentary TiO the amount of soluble salts of silicon and aluminum added to said slurry being such that the total amount of hydrated aluminum silicate deposited on said pigmentary TiO is from 5 percent to 20 percent based on weight of said TiO whereby said pigmentary TiO is completely encapsulated by said hydrated aluminum silicate, said soluble salts being added in proportions to provide a ratio of SiO :Al O in the range from 2:1 to 8: 1, and thereafter finishing the silicate encapsulated TiO by coating with from 1.0 to 2.0 percent hydrous aluminum oxide followed by drying and milling. 

2. A silicate encapsulated TiO2 pigment according to claim 1 wherein the amount of sponge alumina on said silicate encapsulated TiO2 is about 1.5 percent calculated on a TiO2 weight basis.
 3. A silicate encapsulated TiO2 pigment according to claim 1 wherein the silicate encapsulated TiO2 pigment has been dried and milled.
 4. A silicate encapsulated TiO2 pigment according to claim 3 wherein the amount of hydrated aluminum silicate on said TiO2 pigment is from 5-10 percent and the amount of sponge alumina on said silicate encapsulated TiO2 is about 1.0 percent calculated on a TiO2 weight basis.
 5. A method for producing a silicate encapsulated TiO2 pigment characterized by a dense impervious skin of hydrateD aluminum silicate and having exceptionally good color and durability when incorporated in acrylic coating compositions which comprises forming an aqueous slurry of pigmentary TiO2, adjusting the pH of the slurry to fall within the range of from 8.5 to 10.5 and coprecipitating SiO2 and Al2O3 onto the pigmentary TiO2 in two successive stages to form successive coatings of hydrated aluminum silicate on said pigmentary TiO2 the first stage coprecipitation of SiO2 and Al2O3 being effected by heating the slurry to a temperature of at least 70* C. to boiling, simultaneously adding solutions of soluble salts of silicon and aluminum to said slurry at a rate not to exceed 10 percent total silicate coating per hour on a TiO2 weight basis and maintaining the pH of said slurry in the range 8.5 to 10.5 and the temperature of said slurry at least 70* C. to boiling during the addition of said solutions, thereafter adjusting the pH of the slurry in the range of from 7.0 to 10.5 and maintaining the slurry at a temperature of at least 70* C. to boiling for at least 45 to 90 minutes to age the hydrated aluminum silicate coating on said pigmentary TiO2, the second stage coprecipitation of SiO2 and Al2O3 being effected by readjusting the pH of the said slurry in the range of from 8.5 to 10.5 and again simultaneously adding solutions of soluble salts of silicon and aluminum to said slurry in the same manner as that described for the first stage coprecipitation, thereafter readjusting the pH of the slurry at a temperature of at least 70* C. to boiling for at least 45 to 90 minutes to age the second coating of hydrated aluminum silicate, the amount of soluble salts of silicon and aluminum added to said slurry being such that the total amount of hydrated aluminum silicate deposited on said pigmentary TiO2 is from 5 percent to 20 percent based on weight of said TiO2 whereby said pigmentary TiO2 is completely encapsulated by said hydrated aluminum silicate, said soluble salts being added in proportions to provide a ratio of SiO2:Al2O3 in the range from 2:1 to 8:1, and thereafter finishing the silicate encapsulated TiO2 by drying at from 120* C. to 350* C. followed by milling.
 6. A method for producing a silicate encapsulated TiO2 pigment according to claim 5 wherein the temperature of the TiO2 slurry is maintained at 90* C. during both said first and second stage coprecipitations of the alumina and silica and each coating of hydrated aluminum silicate is aged for 45 minutes.
 7. A method for producing a silicate encapsulated TiO2 pigment according to claim 5 wherein the total amount of silica and alumina used in forming said skin of hydrated aluminum silicate is 10 percent on a TiO2 weight basis and the silicate coated TiO2 is dried by heating to a temperature of 120* C., and steam micronized at a steam to pigment ratio of 5:1.
 8. A method for producing a silicate encapsulated TiO2 pigment according to claim 5 wherein the total amount of silica and alumina used in forming said skin of hydrated aluminum silicate is 5 percent on a TiO2 weight basis and the silicate encapsulated TiO2 is finished by drying at a temperature from 120-350* C., steam micronizing at a steam to pigment ratio of from 3:1 to 5:1, and thereafter coated with from 1.0 to 2.0 percent hydrous aluminum oxide on a TiO2 weight basis, followed by drying and steam micronizing.
 9. A method for producing a silicate encapsulated TiO2 pigment accoRding to claim 5 wherein the ratio of silica and alumina used in coprecipitating the hydrated aluminum silicate on said pigmentary TiO2 is 8:1 the total amount of silica and alumina comprises about 4.5 percent on a TiO2 weight basis, and the silicate encapsulated pigment is finished by drying at a temperature of 120* C., steam micronizing at a steam to pigment ratio of 5:1 and thereafter coated with 1.5 percent hydrous aluminum oxide on a TiO2 weight basis followed by drying at 120* C. and steam micronizing at a steam to pigment ratio of 5:1.
 10. A method for producing a silicate encapsulated TiO2 pigment according to claim 5 wherein the ratio of SiO2 to Al2O3 is 4:1 and the application of each successive skin coating is followed by drying and milling.
 11. A method for producing a silicate encapsulated TiO2 pigment according to claim 10 wherein each successive skin coating is finished by drying at a temperature within the range from 120*-350* C. and milling at a steam to pigment ratio of from 3:1 to 5:
 12. A method for producing a silicate encapsulated TiO2 pigment according to claim 11 wherein the ratio of SiO2 to Al2O3 is 8:1 and a coating from 1.0-2.0 percent hydrous aluminum oxide is applied to the silicate encapsulated pigment following drying and milling of the final skin coating after which the alumina coated silicate encapsulated pigment is dried and milled.
 13. A method for producing a silicate encapsulated TiO2 pigment characterized by a dense impervious skin of hydrated aluminum silicate and having exceptionally good color and durability when incorporated in acrylic coating compositions which comprises forming an aqueous slurry of pigmentary TiO2, adjusting the pH of the slurry to fall within the range of from 8.5 to 10.5 and coprecipitating SiO2 and Al2O3 onto the pigmentary TiO2 to form a coating of hydrated aluminum silicate on said pigmentary TiO2 the coprecipitation of SiO2 and Al2O3 being effected by heating the slurry to a temperature of at least 70* C. to boiling, simultaneously adding solutions of soluble salts of silicon and aluminum to said slurry at a rate not to exceed 10 percent total silicate coating per hour on a TiO2 weight basis and maintaining the pH of said slurry in the range 8.5 to 10.5 and the temperature of said slurry at least 70* C. to boiling during the addition of said solutions, thereafter adjusting the pH of the slurry in the range from 7.0 to 10.5 and maintaining the slurry at a temperature of at least 70* C. to boiling for at least 45 to 90 minutes to age the hydrated aluminum silicate coating on said pigmentary TiO2, the amount of soluble salts of silicon and aluminum added to said slurry being such that the total amount of hydrated aluminum silicate deposited on said pigmentary TiO2 is from 5 percent to 20 percent based on weight of said TiO2 whereby said pigmentary TiO2 is completely encapsulated by said hydrated aluminum silicate, said soluble salts being added in proportions to provide a ratio of SiO2:Al2O3 in the range from 2:1 to 8:1, and thereafter finishing the silicate encapsulated TiO2 by coating with from 1.0 to 2.0 percent hydrous aluminum oxide followed by drying and milling. 